Comparative physiology: gas exchange and circulation in fishes.

Abstract

Knowledge about the comparative physiology of respiratory and circula­ tory processes, particularly with reference to the vertebrate animal, has ac­ cumulated at an unprecedented rate during the last decade. The advance­ ment of knowledge has sprung from physiological laboratories in zoology and biology departments, and involvement of physiologists in medical schools has progressively become important. This trend has placed cardio­ respiratory physiology of invertebrates and lower vertebrates in proper con­ text with general and mammalian physiology, and the resulting synthesis and integration of information have emphasized important common princi­ ples governing circulation and respiratory gas exchange. The present review surveys gas exchange and circulation in fishes. Em­ phasis is on aquatic forms, but the special problems of transition to air­ breathing are discussed in the context of air breathing fishes. Problems of gas exchange and internal gas transport in waterbreathing vertebrates have been analyzed in theory and presented graphically by Rahn and others (129, 170, 238, 260). The graphic representation of the exchange process in a P02-PC02 diagram was first introduced for fishes by Willmer (331) and was developed further and used by Rahn to set the limitations of aquatic gas exchange (238). Recently, Jones et al (170) in a theoretical paper offered graphic repre­ sentations relating the ventilation-perfusion ratio in fish to the level of arte­ rial blood oxygen saturation. Assuming constant O2 uptake, they described the determinants of the ventilation-perfusion ratio in terms of the 02-Hb affinity of the blood, the O2 content of blood and water passing to and from the exchange surfaces, and the average P02 gradient across the exchange surfaces. Their graphic representation aids in the prediction of adjustments to hypoxia and exercise. On theoretical grounds Taylor et al (318) developed a computer model